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Date: Mon, 16 May 2022 14:03:20 +0100 From: Mel Gorman <mgorman@...hsingularity.net> To: Hyeonggon Yoo <42.hyeyoo@...il.com> Cc: Dave Hansen <dave.hansen@...el.com>, Tom Lendacky <thomas.lendacky@....com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Tianyu Lan <Tianyu.Lan@...rosoft.com>, "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, vbabka@...e.cz, akpm@...ux-foundation.org, willy@...radead.org Subject: Re: Is _PAGE_PROTNONE set only for user mappings? On Fri, May 13, 2022 at 02:33:38PM +0900, Hyeonggon Yoo wrote: > On Thu, May 12, 2022 at 11:37:48AM +0100, Mel Gorman wrote: > > On Wed, May 11, 2022 at 02:20:45PM +0900, Hyeonggon Yoo wrote: > > > > pgprot_t vm_get_page_prot(unsigned long vm_flags) > > > > { > > > > pgprot_t ret = __pgprot(pgprot_val(protection_map[vm_flags & > > > > (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]) | > > > > pgprot_val(arch_vm_get_page_prot(vm_flags))); > > > > > > > > return arch_filter_pgprot(ret); > > > > } > > > > EXPORT_SYMBOL(vm_get_page_prot); > > > > > > I guess it's only set for processes' VMA if no caller is abusing > > > vm_get_page_prot() for kernel mappings. > > > > > > But yeah, just quick guessing does not make us convinced. > > > Let's Cc people working on mm. > > > > > > If kernel never uses _PAGE_PROTNONE for kernel mappings, it's just okay > > > not to clear _PAGE_GLOBAL at first in __change_page_attr() if it's not user address, > > > because no user will confuse _PAGE_GLOBAL as _PAGE_PROTNONE if it's kernel > > > address. right? > > > > > > > I'm not aware of a case where _PAGE_BIT_PROTNONE is used for a kernel > > address expecting PROT_NONE semantics instead of the global bit. NUMA > > Balancing is not going to accidentally treat a kernel address as if it's > > a NUMA hinting fault. By the time a fault is determining if a PTE access > > is a numa hinting fault or accesssing a PROT_NONE region, it has been > > established that it is a userspace address backed by a valid VMA. > > > > Thanks Mel, and IIUC nor does do_kern_addr_fault() in arch/x86/mm/fault.c > expect _PAGE_PROTNONE instead of _PAGE_GLOBAL. I want to make it clear > in the code that _PAGE_PROTNONE is only used for user mappings. > > How does below look? > I've no strong objections. I worry that this somehow could be used to set PAGE_USER on a kernel mapping page and maybe a comment would be more appropriate. However, I'm failing to imagine how NUMA balancing could be fooled into doing that. -- Mel Gorman SUSE Labs
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